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Windows 10's installer will pull the key from a chip on the motherboard. The manufacturer paid for a license and you can reinstall Windows 10 on the PC without entering the key.

Buy a new PC with Windows 10: If your PC came with Windows 10 installed, it likely has a license key embedded into its UEFI firmware.

A license associated with your PC is stored on Microsoft's servers.

So, if you upgraded when Windows 10 was released and downgraded shortly after, you're still eligible to upgrade to Windows 10 for free. Once you've performed the upgrade once, your PC has a Windows 10 license forever. You can also still install Windows 10 and enter a Windows 7 or 8 key in the installer to receive a free Windows 10 upgrade license.

Upgrade from Windows 7 or 8: Microsoft still offers a free Windows 10 upgrade to PC users who use accessibility tools.

There are a variety of ways you can get a Windows 10 license for your PC, and many of them are still free. Related: All the Ways You Can Still Upgrade to Windows 10 for Free

For windows instal O&O DiskImage Professional 18.4.309